Conventional wisdom has held that seriously mentally ill (SMI) homeless persons generally underutilize public-sector services. However, a growing body of literature is now showing that they, in fact, do use a considerable range of public services (e.g., jails, medical emergency rooms), although not necessarily those services indicated for management of their mental illness (i.e., community mental health clinics). Clearly, our community-based "system" of care is failing to meet the profound and multiple needs of the SMI homeless. This may be due, in part, to the fact that adequate care entails the coordination and integration of a complex range of disparate public services. To understand better how these service systems, as a whole, might become more clinically effective and economically efficient, and how care for the homeless SMI might become more appropriate and humane, we need to know more about the current extent and pattern of SMI homeless persons' service use and magnitude and distribution of associated costs. This study is a first step toward estimating homeless SMI persons' use of mental health, criminal justice, entitlement, and medical services and the costs associated with it. The specific aims are: (I) to determine the extent to which homeless individuals receive services and incur costs across service sectors: (2) to assess the extent of costs associated with inappropriate use; (3) to examine whether more intensive community-based mental health care is associated with lower probability of inappropriate use of other services; and (4) to compare homeless SMI, non-SMI homeless, and domiciled SMI across these same parameters. To address these aims, we will use a combination of primary data collected from a representative sample of homeless persons in Houston, Texas, and secondary data (administrative data sets) from local and state service agencies. Homeless individuals will be linked retrospectively into three years of service-use data from at least sick different agencies to create "profiles of service use" at the individual level. Primary data will provide information on diagnosis and other relevant clinical, demographic and lifestyle variables. Secondary data will provide accurate accounts of service use across multiple systems of care.